In a tactical retreat that could have important ramifications, Nigel Farage, the leader of Britain’s populist Brexit Party, on Monday promised not to run candidates in areas held by the ruling Conservatives. The move was expected to bolster Prime Minister Boris Johnson’s prospects in a topsy-turvy general election next month.
Mr. Farage had been under pressure from his own supporters who worried that, by splitting the pro-Brexit vote, he might deprive Mr. Johnson of the victory he needs to pull Britain quickly out of the European Union.
However, the Brexit Party is still planning to fight seats currently held by the opposition Labour Party, and in doing so could draw away Conservative voters in the electoral battlegrounds of the middle and north of England. The Conservatives desperately need to grab Labour seats in areas there that voted Leave in the 2016 referendum on whether to remain in the European Union.
So, while the partial pact helps the Conservatives, analysts do not think it will do so enough to decide the outcome of an election likely to determine whether Britain leaves the European Union at the end of January, as Mr. Johnson wishes, or holds another Brexit referendum next year.
Mr. Farage, who formerly led the U.K. Independence Party, has failed seven times to win a seat in the British Parliament, and said earlier this month that he would not make an eighth attempt himself, but would instead campaign in support of candidates around the country.
That will now be in significantly fewer parliamentary constituencies.
“The Brexit party will not contest the 317 seats the Conservatives won at the last election,” Mr. Farage said in Hartlepool, a struggling town in the northeast of England that voted to leave the European Union in the Brexit referendum.
“We will concentrate our total effort into all the seats that are held by the Labour party,” he said.
The Brexit Party will also take on smaller parties like the Liberal Democrats that oppose withdrawal from the European Union, he said.
Britain operates a winner-takes-all system for general elections with members of Parliament elected when they have the highest vote in a constituency.
Mr. Farage had threatened to contest around 600 seats but on Monday said that he had been reassured by a “change of direction” from Mr. Johnson, who has ruled out any extension, beyond the end of 2020, of talks on future trade ties with the European Union.
Most analysts predict that it would take significantly longer to put in place a trade agreement, and Britain has retained the right to extend the negotiations. This would mean staying under European Union trade rules and paying into the bloc.
Though it helps Mr. Johnson, it is unclear how big a difference the Brexit Party leader’s new tactic will make.
Analysts believe it could allow the Tories to hold more of the seats in south and west of the country where the centrist, anti-Brexit Liberal Democrats are challenging. The worry for Mr. Johnson was that, in these areas, the Brexit Party would siphon off some of his Conservative voters, providing the Liberal Democrats with a path to victory.
But Mr. Farage will not help Mr. Johnson in the crucial electoral battleground, the so-called red wall of seats in the middle and north of England where a majority voted for Brexit. In these areas, where the Labour Party is strong, the Conservatives need to make gains — and Mr. Farage’s Brexit Party gives traditional Labour voters who want to leave the European Union an alternative to the Tories.
It was never likely that Brexit Party, formed earlier this year, would have been able to make a credible challenge all around the country. The tactical retreat helps it concentrate its firepower.
Mr. Johnson told Sky News he had “absolutely not” made a deal with Mr. Farage, but did say he was “glad that there is a recognition that there is only one way to get Brexit done, and that is to vote for the Conservatives.”
Critics seized on the Brexit Party’s announcement to suggest that the two parties were now umbilically tied.
“A vote for Boris Johnson’s Conservatives is a vote for Nigel Farage’s politics,” wrote Keir Starmer, Labour’s Brexit spokesman, on Twitter.
Nick Boles, a lawmaker who was expelled from the Conservative Party for opposing Mr. Johnson’s Brexit policy, wrote that “there are millions of Conservative Remainers who, like me, were willing to back a soft Brexit. How will they react on discovering that Johnson’s Brexit deal is hard enough for Nigel Farage?”
Ducking out of a national campaign means the Brexit Party can expect less media coverage than its bigger competitors. And Mr. Farage’s decision could increase pressure for anti-Brexit parties to increase their cooperation.
So far, the smaller anti-Brexit parties have agreed to work together in a limited number of regions, though Labour — which is not committed to remaining in the European Union but wants a second referendum — has stood aloof.
Patrick English, an associate lecturer at the University of Exeter, said Mr. Farage’s decision left an uncertain picture.
“Brexit Party candidates standing in Labour-held Conservative targets will make it much harder for Johnson to flip them,” he said. “Therefore, Johnson’s quest for a majority still remains very much up in the air despite this announcement.”